VOCs are a wide ranging class of compounds including environmental pollutants such as certain components of car exhaust gases, solvents and aerosol gases, but also including a range of compounds that are derived from organic matter. One example of a VOC derived from organic matter is ethylene, a plant hormone that causes ripening, whilst another example is trimethylamine, a gas commonly given off by fish as it decomposes.
The removal of VOCs derived from organic matter is of interest for a variety of applications. The adsorption of ethylene can prevent undesired ripening and softening, loss of colour, loss of leaves and sprouting to occur in fruit and vegetables, and it is also known to prevent other food and horticultural products from perishing prematurely, and can help eliminate unpleasant smells.
Various methods have been used to oxidise or combust VOCs using Pt on Al2O3 or KMnO4. However, although these systems are efficient for the removal of VOCs, they have disadvantages associated with their use. Pt on Al2O3 works by catalytically combusting the ethylene at elevated temperatures; therefore, Pt on Al2O3 needs to be used in a heated unit separate from the source of the VOCs (see for example GB 2 163 637 A and U.S. Pat. No. 4,331,693). KMnO4 cannot remove VOCs efficiently from humid environments (see Example 4). Since organic matter, such as food, cannot be heated without being altered and inherently exudes moisture, such systems are unsuitable for use in removing VOCs derived from organic matter.
Other methods used to remove VOCs are suited for use at lower temperatures; these include the use of high surface area supports, usually in conjunction with a promoter, for the adsorption of VOCs. For example, JP 2-261341 discloses the adsorption of ethylene from refrigerated storage compartments, JP 2-233381 discloses an ethylene adsorption film and JP 2000-004783 discloses a combined ethylene adsorber, deodoriser and anti-bacterial product for use in a refrigerator. Specific support materials are not disclosed in any of these publications, instead activated carbon and metal oxides are stated as being generally suitable for use as supports. GB 2 252 968 A relates to an adsorber comprising a sepiolite in combination with a zeolite, and optionally a metal selected from the platinum group metals, the iron group metals, group I metals, group VII metals and the rare earth metals. The most preferred zeolites for use in the invention described in GB '968, are silicalites because their alumina content is almost zero.